Tubular oven

ABSTRACT

A thermal processing plant has heating tubes which are arranged in an undulating line in an oven chamber. Burners which generate a gas stream directed against the opposite wall of the oven are positioned at each of the “wave crests” and in each of the “wave troughs.” The thermal radiation of the oven wall and of the reaction zone (combustion heat) heats the heating tubes uniformly.

[0001] The invention pertains to a heating system, particularly a thermal processing plant, in which a stream of products in tubes is heated indirectly.

[0002] In thermal processing plants, fluids that are often under pressure are heated indirectly, wherein thermal energy is used for heating, a phase change, as reaction heat (endothermic reaction), or for several of these effects. The fluids are guided in heating tubes that are usually loaded up to their thermal limit. In this way, there are no local points of overheating.

[0003] Thermal processing plants 1 that are known from practice are illustrated in FIGS. 7 and 8. The thermal processing plants 1 each have an interior space enclosed by an oven wall 2, e.g., made from bricks, in which heating tubes 4 are arranged. For heating these tubes, burners 6 are positioned in openings in the oven wall 2. In the embodiment according to FIG. 7, the heating tubes 4 are arranged in line vertically and one after the other, so that an alley 8 is formed between pairs of tube lines 7. The burners 6 impart an axial output momentum to the fuel and to the air and fire into the alleys 8, wherein the heat is transferred predominantly through flame and gas radiation to the heating tubes 4. The radiation originating from the oven wall 2 meets the heating tubes 4 non-uniformly.

[0004] This results in a relatively minimal heat transfer effect. In addition, this arrangement is not suitable for the use of flameless-oxidation burners, which form relatively large areas of reducing atmosphere that is harmful to the heating tubes 4.

[0005] With the thermal processing plant 1 according to FIG. 8, the heating tubes 4 are arranged in a line approximately in the center of the interior space 3. Swirl burners 6′ are positioned in openings in the oven wall 2 and they impart an essentially radial output momentum to the fuel and to the supplied combustion air. The flames and heated regions of the oven wall 2 generate a hot spot that transfers the major portion of the energy from the flame to the heating tubes 4 through radiation. The zones of higher radiation intensity are small, and for this reason, a plurality of swirl burners 6′ must be installed.

[0006] Starting from here, the task of the invention is to create an arrangement that allows uniform tube heating while preventing points of overheating in the tubes. In this way, an arrangement that is as compact as possible can be achieved.

[0007] Through the arrangement of burners and heating tubes according to the invention, the burners generate a large zone of increased temperature at the opposite wall and this zone is in radiation exchange with the heating tubes with no shadow formation. The energy is transferred uniformly through solid body radiation (wall—tube), gas radiation (reaction zone—tube), and convection (completely reacted combustion products—tube).

[0008] The burners preferably impart a large axial output momentum to the output fuel-air stream. Preferably, this momentum is so large that the stream reaches the opposite wall and generates a hot spot/region at that location. The burners can have fuel-air-mixed output openings or separate output openings for the fuel and air. This releases heat radiation to the heating tubes.

[0009] If burners that operate according to the principle of flameless oxidation are used, then no temperature spikes are formed near the burner, so that tube line sections opposite in angle can be assembled relatively close in front of the burner. The large-area recirculation of the combustion gases enables, particularly for flameless oxidation, an optimum reduction of the NO_(x) emission even for highest pre-warming of the air. Relatively large, self-forming reducing areas do not contact the heating tubes due to the tube arrangement. In addition, the tube arrangement enables a compact system construction and high heat-transfer effects without temperature spikes at the heating tubes.

[0010] In addition, burners suitable for flameless oxidation also allow the use of problematic fuels or compositions of changing fuels that would conventionally result in stability problems for the formation of a flame.

[0011] Additional details of advantageous embodiments of the invention are the objects of the drawing, the description, and the subordinate claims.

[0012] Embodiments of the invention are illustrated in the drawing. Shown are:

[0013]FIG. 1, a thermal processing plant with burner and heating-tube arrangement according to the invention in a schematic, horizontal, sectional representation,

[0014]FIG. 2, a modified embodiment of the thermal processing plant in horizontal, sectional representation,

[0015]FIG. 3, the thermal processing plant according to FIG. 1 or FIG. 2 in a schematic, perspective representation,

[0016]FIG. 4, a modified embodiment of the thermal processing plant,

[0017]FIG. 5, another modified embodiment of the thermal processing plant, and

[0018]FIG. 6, a burner of the thermal processing plant in a schematic, sectional representation as an illustration of the principle.

[0019] In addition, FIGS. 7 and 8 show thermal processing plants according to the state of the art.

[0020] In FIG. 1, a thermal processing plant 1 is illustrated with an oven wall 2 (2 a, 2 b) that is built from bricks and that defines an oven chamber 3 and forms a wall arrangement. In the oven chamber 3 there are heating tubes 4 which carry a fluid for heating or for maintaining a high temperature. In this way, the fluid can be subjected to chemical changes along its path through the heating tubes 4.

[0021] The heating tubes 4 are axis parallel, as FIG. 1 shows, and arranged at intervals from each other in a zigzag line 10. In this way, every tube line section forms groups 11, 12 of heating tubes 4, which are not parallel to each other and which form an acute angle preferably different from zero (e.g., 30° . . . 80°) with each other. The angles formed by the line sections 11, 12 open in an alternating way towards one and then towards the other of the two opposite oven walls 2 a, 2 b. For the embodiment according to FIG. 1, the sections of the line formed by the groups 11, 12 are straight, wherein the sections of groups 11 are parallel to each other. Also the line sections of the groups 12 are straight and parallel to each other in this example. However, it is also possible to arrange the heating tubes 4 so that the line sections of the groups 11 and 12 are bent or undulating.

[0022] For heating the heating tubes 4 there are burners 6 a, 6 b that are positioned in corresponding openings of the oven wall 2 a and 2 b, respectively. For the embodiment according to FIG. 1, the burners 6 a, 6 b are burners that are suitable for flameless oxidation. They generate a fuel-air stream, i.e., a flow 14 a, 14 b with axial output momentum that is directed against the corresponding opposite oven wall 2 b, 2 a. At the burner outlet, the stabilization of the flame is prevented by means of appropriate design according to flow technology.

[0023] The flow 14 a, 14 b output from the burners 6 a, 6 b is located between two adjacent heating tubes 4, where one tube belongs to group 11 and the other tube belongs to group 12. Then a large-area recirculation flow is formed as illustrated in FIG. 1 by arrow 15, wherein a large-area reaction zone 16 is produced between the line sections of the groups 11, 12.

[0024] The burners 6 a, 6 b are configured as recuperator burners or regenerator burners that permit high air pre-warming. As an example, a principle schematic of a recuperator burner 6 is given in FIG. 6. This has a central fuel supply line 17 that can output fuel with axial output momentum. In addition, the burner 6 has an air supply 18 that likewise imparts axial output momentum to the output combustion air. The air supply 18 is in heat exchange via a recuperator tube 19 with an exhaust stream that is guided in the opposite direction over an exhaust line 20 out of the oven chamber. The recuperator tube 19 is a heat exchanger. Through the discharge of exhaust gasses through the burner 6, temperature regulation zones are minimally influenced on opposite sides.

[0025] As FIG. 3 illustrates, several burners 6 can be arranged in a line on each oven wall 2 a, 2 b, wherein the burners 6 (6 a, 6 b) each generate output momentum directed perpendicular to the heating tubes 4 and are arranged in groups in lines that are parallel to the heating tubes 4. The burners 6 a can be arranged at uniform intervals or at different intervals. For example, for a downward-directed fluid stream there can be a higher number of burners (burner density) in a first zone in order to heat the input fluid stream quickly at first, wherein in subsequent zones 2, 3 a correspondingly fewer number of burners 6 is sufficient in order to maintain the achieved temperature.

[0026] There can be additional burners 6′ that are directed parallel to the heating tubes 4 and thus impart output momentum directed parallel to these heating tubes 4. These burners are preferably positioned in a zigzag arrangement above or below, respectively, the approximately triangular areas 16 of the oven chamber 3 defined by the groups 11, 12 (FIG. 1). They can be used at the startup of the plant, e.g., for initial heating, in which they fire parallel to the heating tubes 4 and later are turned off, switched to flameless oxidation, or operated further. In one preferred embodiment, the burners 6′ are equipped with flame retention baffles and they generate a flame, while the burners 6 a, 6 b are burners without flame retention baffles and they are suitable for flameless oxidation and are turned on, e.g., first in the heated state. If necessary, however, the burners 6 a, 6 b can also be changeable burners that operate at first with flames for ignition and when the required temperature is reached, switch to flameless operation.

[0027] The thermal processing plant 1 described so far operates as follows:

[0028] In operation, the burners 6 a, 6 b release a fuel-air stream that is directed towards the opposite oven wall 2 b, 2a perpendicular to the heating tubes 4 and approximately perpendicular to the oven wall 6 a, 6 b. The stream passes between adjacent heating tubes 4 and reaches the oxidation zones 16, where flameless oxidation occurs. The heat released here is led with the gas flow to the oven wall 2 b, 2 a, where a corresponding, strongly heated area 21 is formed. The heat radiation output from this region 21, and also the heat radiation output from the oxidation region 16, meets the heating tubes 4 without shadowing. The heat distribution is relatively uniform, i.e., all heating tubes 4 obtain approximately the same amount of heat.

[0029] By means of the alternating, oppositely directed momentum from the burners 6 a, 6 b there results a large-area circulation, as indicated by the arrow 15. The large-area recirculation includes the heating tubes 4 of each group 11, 12, which achieves a uniform, convective heat transfer. The heating tubes 4 lie outside the oxidation zones 16, where the reducing atmosphere prevails, which counteracts corresponding tube wear and tear.

[0030] With the embodiment illustrated in FIG. 1, the heating tubes 4 are generally positioned relative to each other at equal intervals. By means of the number of heating tubes, their intervals, and the angles formed by the sections of the groups 11, 12 with each other, the heat distribution on the heating tubes 4 can be selectively influenced. By means of the angle, the number, and the interval of the heating tubes, the relationship of the area of the oven wall 2 to the tube surface can be determined, where the preferred goal is approximate uniformity between surface area of the tube and of the oven wall.

[0031] The burners 6 can preferably be adjusted and turned on and off individually. In this way, the heat distribution in the oven chamber 3 can be additionally selectively influenced.

[0032] In FIG. 2, a modified embodiment of the thermal processing plant 1 according to the invention is illustrated. Instead of burners suitable for flameless oxidation, here there are burners 6 that generate a flame directed towards the opposite oven wall 2 a, 2 b. The zigzag arrangement of the heating tubes 4 and the large area flow relations, however, are similar to those for the embodiment according to FIG. 1. Likewise, there can be additional burners 6′, as indicated in FIG. 3.

[0033] As FIG. 4 illustrates, it is also possible, both with burners with flames and also with flameless burners, to arrange the heating tubes 4 in an undulating line. In turn, this line also has sections that are formed of groups 11, 12 of heating tubes 4 and that are not directed parallel to each other. However, in agreement with the embodiment according to FIGS. 1 and 2, the groups 11, 12 preferably have an equal number of heating tubes 4, wherein the intervals between the heating tubes 4 of group 11 and the heating tubes 4 of group 12 increase from the corresponding burner 6 up to the opposite oven wall. The increase in the interval does not have to be linear. Likewise, the intervals between the heating tubes 4 do not have to be equidistant.

[0034] As FIG. 5 illustrates, if necessary, there can be additional heating tubes 4′, which are used, e.g., as overheating tubes, which are assigned to another processing step, or which are otherwise subjected to fundamentally different conditions than the heating tubes 4. However, the additional tubes 4′ should be arranged so that they do not significantly influence the described operation of the thermal processing plant 1.

[0035] A thermal processing plant 1 has heating tubes 4 which are arranged in an undulating line in an oven chamber. Burners which generate a gas stream directed against the opposite wall of the oven are positioned at each of the “wave crests” and in each of the “wave troughs.” The thermal radiation of the oven wall and of the reaction zone (combustion heat) heats the heating tubes uniformly. 

1. Heating system, particularly thermal processing plant (1) with a wall arrangement (2) that at least partially defines a heatable interior space (3), with heating tubes (4) that are used for guiding one or more fluids for heating and that are arranged in the interior space (3) essentially parallel to each other, with burners (6) that are arranged for heating of the interior space (3) perpendicular to the heating tubes (4) and that are directed towards an opposite section (21) of the wall arrangement (2), wherein the heating tubes (4) are arranged in a line (10) that has at least two sections (11, 12) that each open up away from the burners (6).
 2. Heating system according to claim 1, characterized in that the tubes are arranged at intervals to each other on a zigzag, undulating, or meandering line (10) that stretches between two parallel walls (2 a, 2 b) of the wall arrangement (2).
 3. Heating system according to claim 1, characterized in that the burners are aligned so that the gas stream output from each burner (6) is directed between two adjacent heating tubes (4) into an essentially free region of the interior space.
 4. Heating system according to claim 1, characterized in that several burners (6) directed perpendicular to the heating tubes (4) are arranged in a group (11, 12), in which they are each arranged in a line and parallel to each other.
 5. Heating system according to claim 1, characterized in that a few burners (6″) are aligned essentially parallel to the heating tubes (4).
 6. Heating system according to claim 1, characterized in that the burners (6) are provided with a device for air pre-warming.
 7. Heating system according to claim 1, characterized in that the burners (6) are aligned for forming conditions that permit flameless oxidation of the fuel input into the interior space (3).
 8. Heating system according to claim 1, characterized in that the burners impart a predominantly axial output momentum with minimum or no swirling to the fuel directed into the interior space (3) and to the combustion air. 